A key aspect of Einstein’s general theory of relativity (GR) has been tested using the strongest gravitational field so far. The measurement was made by observing changes in optical absorption lines of a star orbiting close to Sagittarius A* – the supermassive black hole at the centre of the Milky Way.

The work was done by physicists working on the GRAVITY Collaboration, which uses the Very Large Telescope at the European Southern Observatory in Chile.Since it was first proposed in 1915, GR has stood firm against every experimental challenge that physicists have come up with. Many of these tests have focused on an important tenet of GR called the Einstein equivalence principle (EEP). Detecting a breakdown of the EEP could point towards new physics beyond GR and could provide important clues about how to develop a quantum theory of gravity.Different systemsOne key element of the EEP is local position invariance (LPI), which says that local nongravitational measurements on a system must be the same, no matter where they are measured in space-time. LPI has been tested by comparing the properties of two different systems as they both experience the same change in gravitational potential. If LPI holds, ...